Effective acuity and refraction targets

ABSTRACT

A viewing target for a visual acuity and refraction measurement includes at least one line comprising a width dimension that is below a resolution limit width (hereinafter “RLW”) of a test subject visual acuity, and an adjustable length dimension that is initially set at greater than the RLW of the test subject visual acuity. A base, at least approximately intersecting the line, has a thickness along the direction of the length of the line that is greater than the RLW of the test subject visual acuity. The length dimension of the line is adjustable in increments small enough to effectively approximate the visual acuity of the test subject by determining a shortest resolvable line and a next smaller line that is not resolvable by the test subject.

PRIORITY

This application is a Continuation of U.S. patent application No.14/616,738, filed Feb. 8, 2015, now U.S. Pat. No. 9,247,871; which is aContinuation of U.S. patent application No. 14/158,924, filed Jan. 20,2014, now U.S. Pat. No. 8,950,865; which is a Continuation of U.S.patent application No. 12/947,694, filed Nov. 16, 2010, now U.S. Pat.No. 8,632,183; and is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The most common refraction target used by optometrists orophthalmologists today is the Snellen letter eye chart. It consists of alist of letters typically arranged in a row, or a column. The lettersize is also linked to the visual acuity level. For example, letters offont size having the height measurement of the letter N or H of 8.9 mm,it is commonly used to measure a level of 20/20 visual acuity, when suchletters are placed at 20 feet away from the test subject. For the 20/10acuity letters, the letter size is scaled by a factor of two smaller. Onthe other hand, the 20/40 letters would have twice the size of those of20/20, etc.

Such eye chart is popular among eye care professional due to its lowcost, easy to make, and it works for most purposes. However, a drawbackof such type of eye chart is that patients can memorize the letters.Also, each letter has a different effectiveness, that is, some lettersare easier than others to read. Moreover, the letters have their ownintrinsic orientation. Each letter includes a certain uniqueconfiguration of “strokes” presenting a directional preference. Forexample, the letter H favors the vertical direction, and E has threehorizontal lines and one vertical line, etc. Another drawback is thatthe letters are in the English language, such that the test results maydiffer depending on the fluency of the test subject with the Englishlanguage.

Other types of eye charts include tumbling E and Landolt C. Both offercertain benefits, however, neither are used as a refraction target,because they contain only E's and C's, respectively. Therefore, bothrequire an additional directional response to indicate if a test subjectcan identify the orientation of a symbol correctly. For a quick check ofastigmatism, there is yet another type of chart typically used toindicate the existence of cylinder refractive error. It consists of linepairs, or thick lines, typically 12 pairs arranged in 30 degreeincrements, like the spokes of a wheel. A typical line width in anastigmatism chart is about 2 mm or greater. A thick line or line pairsmeet at or near the center of the spoke pattern. In some cases, thespokes stop when they touch or merge with the next spoke or otherwiseleave a small central blank zone. Since astigmatism refractive errorsare asymmetric in nature, it causes a non-uniform appearance of thespokes, that is, some spokes appears darker than others. A conventionalastigmatism chart does not provide visual acuity level information. Itis hardly used as a refraction target, nor does it have the sensitivityof a Snellen eye chart when used as a refraction target.

It is desired to have a refraction target, or an eye chart, that isuniversally usable for all ages from child to adult, without languagebarriers, for literate or illiterate, convenient, low cost, and easilyavailable to eye care professionals, as well as having no directionalpreference (unless it is specifically desired to test for cylindererror). Since a desired viewing target is to be used not only for acuitymeasurement, but also as a refraction target, to arrive at a moreaccurate refraction end point, it is also desired that such targetexhibit higher discrimination sensitivity compared to a Snellen orletter eye chart.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a viewing target including a line and abase in accordance with certain embodiments.

FIGS. 2(A)-2(C) schematically illustrate the viewing target of FIG. 1with an adjustment line length.

FIG. 3(A) schematically illustrates a rotationally-symmetric viewingtarget including multiple lines and a circular base in accordance withcertain embodiments.

FIGS. 3(B)-3(D) schematically illustrate the viewing target of FIG. 3with adjustment line lengths.

FIG. 4(A) schematically illustrates a viewing target in accordance withcertain embodiments including a semi-circular base connected to multiplelines.

FIG. 4(B) schematically illustrates a viewing target in accordance withcertain embodiments including a circular base connected to multiplelines.

FIGS. 5(A)-5(D) schematically illustrate elongating a viewing target andshortening the lines of the viewing target in accordance with certainembodiments to measure residual cylinder error.

FIG. 6 schematically illustrates moving a line around a circular base todetermine an axis angle of a cylinder error in accordance with certainembodiments.

FIGS. 7, 8 and 9 schematically illustrates alternative viewing targetswith non-circular bases in accordance with certain alternativeembodiments.

DETAILED DESCRIPTIONS OF THE EMBODIMENTS

A viewing target is provided on a display for visual acuity andrefraction measurements of a test subject. The viewing target includes abase and at least one line. The line has a width dimension that is belowa resolution limit width (hereinafter “RLW”) of a test subject visualacuity, and an adjustable length dimension that is initially set atgreater than the RLW of the test subject visual acuity. The base atleast approximately intersects the line, and has a thickness along thedirection of the length of the line that is greater than the RLW of thetest subject visual acuity. In performing a visual acuity and refractionmeasurement of a test subject, the length dimension of the line isadjustable in increments small enough to effectively approximate thevisual acuity of the test subject by determining a shortest resolvableline and a next smaller line that is not resolvable by the test subject.

The increments of the adjustable length of the line may be as small as asingle display pixel or a selected multiple thereof.

The line and the base may be white or gray color, or a combinationthereof, and a background against which the target is displayed may beblack color, or vice-versa.

The colors and/or grayscale characteristics and/or brightness of theline and base and background against which the target is displayed maybe selectable.

The width of the line may be less than 20/10 RLW.

The length of the line may be greater than 20/40 RLW.

The thickness of the base may be equal to or greater than 20/40 RLW.

There may be a single line or multiple lines differing in their lengthor direction, or both. The base may include a polygon shape and havelines protruding from it.

Five lines may protrude from the base angularly spaced at approximatelyequal angles apart, wherein the angles range from 2 degrees to 45degrees.

Each of the lines may be equal in length and adjustable.

The base may have a circular shape.

The target may be adjustably elongatable for testing cylinder error.

A contrast level of the at least one line may be adjustable in a rangebetween 1% and 100%.

The viewing target may be displayable on a computer monitor, or otherelectronic screen.

The base may have a circular shape, and the viewing target is adjustablefor displaying in temporal increments a line protruding from the base atdifferent angles. The different angles comprise increments between 2degrees and 45 degrees. The temporal increments comprise 0.01 seconds to2 seconds.

The line and base comprise a same color.

A method of determining a visual acuity level of a test subject infurther provided, including reducing a length of at least one line in aviewing target until the test subject indicates that the line hasdisappeared into a base. The viewing target may include at least oneline having a width dimension that is below a resolution limit width(hereinafter “RLW”) of a test subject visual acuity, and an adjustablelength dimension that is initially set at greater than the RLW of thetest subject visual acuity. The base may at least approximatelyintersect the line, and have a thickness along the direction of thelength of the line that is greater than the RLW of the test subjectvisual acuity. In performing a visual acuity and refraction measurementof the test subject, the length dimension of the line is adjustable inincrements small enough to effectively approximate the visual acuity ofthe test subject by determining a shortest resolvable line and a nextsmaller line that is not resolvable by the test subject.

Another method of determining a refraction error includes determining aquality of an image of a line of a viewing target. The viewing targetincludes at least one line comprising a width dimension that is below aresolution limit width (hereinafter “RLW”) of a test subject visualacuity, and an adjustable length dimension that is initially set atgreater than the RLW of the test subject visual acuity. A base at leastapproximately intersects the line and has a thickness along thedirection of the length of the line that is greater than the RLW of thetest subject visual acuity. In performing a visual acuity and refractionmeasurement of the test subject, the length dimension of the line isadjustable in increments small enough to effectively approximate thevisual acuity of the test subject by determining a shortest resolvableline and a next smaller line that is not resolvable by the test subject.

A further method is provided for determining a refraction error. Themethod includes reducing a length of at least one line in a viewingtarget until a test subject indicates that the line has disappeared intoa base. The viewing target includes at least one line having a widthdimension that is below a resolution limit width (hereinafter “RLW”) ofa test subject visual acuity, and an adjustable length dimension that isinitially set at greater than the RLW of the test subject visual acuity.The base at least approximately intersects the line and has a thicknessalong the direction of the length of the line that is greater than theRLW of the test subject visual acuity. In performing a visual acuity andrefraction measurement of the test subject, the length dimension of theline is adjustable in increments small enough to effectively approximatethe visual acuity of the test subject by determining a shortestresolvable line and a next smaller line that is not resolvable by thetest subject.

There may be a single line or multiple lines differing in their lengthor direction, or both. The base may be circular or have a polygon shape,wherein each of the lines may protrude from the base. For example, theremay be five or seven lines or eight lines or more protruding from thebase angularly spaced at approximately equal angles apart, wherein theangles may range from 2 degrees to 45 degrees. Each of the lines may beequal in length and adjustable. The target may be adjustablyelongatable. An example of an elongated target is to take FIG. 3(A) andto change its aspect ratio along either the x, or the y direction, suchthat the elongation is along x or y axis in this example. Generallyspeaking, the elongated direction can be along any direction betweenzero to 180 degrees. Such elongated target can be used for testingcylinder errors.

A tangible processor-readable medium is also provided having storedtherein program code for programming a processor to generate any of theviewing targets described herein on a display for visual acuity andrefraction measurement of a test subject.

A visual acuity measurement system is also provided including a displayfor showing any of the viewing targets described herein. A machine maybe coupled with the display and configured to adjust the viewing targeton the display during a visual acuity and/or refraction measurement of atest subject. A calculator or look-up table, or both, may be used todetermine the visual acuity of the test subject based on the visualacuity measurement.

Length Units RLW Measured in Visual Acuity Resolution Limit

An eye chart consists of letters which are arranged in rows. Letters indifferent rows typically have a different font size. However, such eyechart can be used at various viewing distances. The distance at whichthe eye chart is placed from the test subject determines the acuitylevel of certain letters. In other words, for a given or fixed acuitylevel, the length dimension of the letters in the viewing target isinherently related to the viewing distance. As an example, at 20 feetviewing distance from a test subject, a letter E with height dimensionof 8.87 mm is typically used as a standard size for a 20/20 acuitylevel. The separation of the three horizontal lines in the letter E isto be set at about 1.77 mm, and the width of the vertical line and thehorizontal lines are to be set to 1.77 mm as well, which is also thewidth subtended by 1 minute of arc at a distance of 20 feet, adefinition of 20/20 spatial resolution. Hereby, we define a measure ofspatial resolution limit in units of visual acuity. For example, 20/20has spatial resolution limit width of 1.77 mm at 20 feet viewingdistance, so a new unit 20/20 RLW means 1.77 mm when the target isplaced at 20 feet viewing distance. This newly defined length unit isused throughout the rest of this specification. In another example, ifthe viewing distance is changed to 10 feet, the 20/20 RLW thencorresponds to 0.89 mm. If the same letter E with the height of 8.8 mmis used at 10 feet, that E letter has now changed to a standard formeasuring 20/40 visual acuity level. Since refractive targets are oftenused to assess the visual acuity level, it is convenient to use a lengthdimension in the units of acuity RLW in the rest of this patentspecification. Otherwise, whenever a length dimension is called out, theviewing distance would also be specified. A RLW unit has a built infunction for the viewing distance, in the sense that it scales with theviewing distance, just like how a visual acuity eye chart is used. Wealso have the following convention: if a test subject has a visualacuity of 20/20, that subject's RLW is 1.77 mm at 20 feet viewingdistance. By the same token, if the viewing target is placed at 10 feetfrom the test subject, a 20/20 RLW means 0.885 mm. Also, f a testsubject has a visual acuity of 20/20, the RLW of this test subject is1.77 mm at 20 feet viewing, 0.885 mm at 10 feet viewing, or 2.66 mm at30 feet viewing, etc.

Refraction and Acuity Measurement

Refracting a test subject involves finding or identifying the correctend points for the sphere, cylinder, and/or the axis orientation thatwould provide the best possible corrected vision. Therefore, it isdesired to determine the effect on the vision of any residual refractiveerror in any one of the three key refraction end points. Sphericalerrors typically induce a symmetrical blur of any part of an image. Apure cylinder error, however, has a preferential direction. An imagebefore the correction appears to be elongated along the axis directionif intended corrective cylinder is positive, and appears to beperpendicular to the axis direction if it is negative correctivecylinder. If the refractive error involves both spherical andcylindrical components, then the resultant image formed at the retinawould have the combined effects of spherical blur and a preferentialelongation.

A refractive target in accordance with certain embodiments includes thinand narrow black lines, and a base. Certain dimensions of the line andthe base are described in various embodiments.

An example of a viewing target in accordance with certain embodiments isshown in FIG. 1. A thin line 110 has a length dimension substantiallygreater than its width. The width is chosen to be less than that of thespatial resolution limit of the eye for which the visual acuity isintended to be measured. For example, the width of the thin line can beset to 20/20 RLW, if it is used to measure visual acuity level of 20/30or worse acuity. Since most people cannot see better than 20/10 level,it is safe to choose the width of a thin line in the inventive target tobe equal to or less than 20/10 RLW (0.89 mm at 20 feet viewing, or 0.445mm at 10 feet viewing) for a target to be used for a general populationtesting.

The base 120 has a thickness dimension 130, which is also configuredadvantageously in accordance with multiple embodiments. The basethickness is chosen in certain embodiments to be greater than visualacuity limit of the test subject. For example if a test subject has avisual acuity level of 20/15 or better, the base thickness may be set atequal to or greater than 20/15 RLW (1.33 mm at 20 feet viewing).

In accordance with certain embodiments, the width if the thin line issmaller than the test subject's RLW. A line width can be selected, forexample, that is smaller than anyone's RLW in human population, such asa 20/10, or 20/8. Also, if there is a record of what a certain patientcan see roughly from a previous eye exam, then the line width can beselected in accordance with the last known RLW of the test subject, atleast as a starting point.

Acuity Testing

Another advantageous feature in accordance with certain embodiments isthat the length of the thin line is adjustable. In one embodiment, thelength of the thin line can be used to gauge the visual acuity of a testsubject. One method to test the visual acuity level is to provide a longthin line attached to a base, e.g., a dark base such as a black or bluebase, for the test subject to look at. In an example, as schematicallyillustrated in FIG. 2(A), the line width is set at less than 20/10 RLW,and the line length is set at 20/60 RLW. Since most normal functioningtest subject's have vision that is better than 20/40, a test subjectwith such visual acuity should be able to see the thin line and the darkbase. Next, the thin line is shortened as illustrated in FIG. 2(B). Thenit is further shortened as shown in FIG. 2(C), where the test subjectcan no longer distinguish the existence of a line, or a “small tickmark” above the black base. At this point, the thin line appears to thetest subject to have disappeared into the base. The RLW line length whenthis happens is a measure of the test subject's visual acuity. Forexample if the thin line disappears as the line is shortened to 20/15RLW, the test subject has a visual acuity level of 20/15 or worse. Theincrement of adjustability for the line length is approximatelycontinuous in accordance with human eye resolvability, and in practicewould be limited by the display capacity of the monitor, e.g., theincrement could be one pixel on the display. For higher resolutiondisplay, the increment could be two pixels or more and the increment canstart out higher and be more finely adjusted as the minimum resolvablelength is reached by a test subject.

An advantage of using a viewing target in accordance with certainembodiments for acuity testing is that it is a subjective and fast testthat can be easily self administrated. In one embodiment, the testsubject can use a knob to change the length of the thin line above. Thetest subject is asked to turn the knob until the line disappears intothe black base, stopping at exactly when it disappears. Then, the testsubject can push a second button to indicate the test is finalized. Avisual acuity result can then be printed out, stored or displayed on ascreen for record keeping. The test subject does not need to readEnglish letters. This target can be used by illiterate subject's, andacross all languages.

As it is useful to set a generally usable width of the thin line for ageneral population in an example above, the base can be chosen to makesuch a viewing target useful, for a range in VA in test subjects from20/10 to 20/100, for example, if the base thickness is greater than20/100 RLW (or 8.9 mm at 20 feet viewing).

To eliminate the dependency of directional dependency, a viewing targetin accordance with certain embodiments may be constructed with a seriesof multiple thin lines, extending from a base, in this example a rounddisk, or a circular area as shown in FIG. 3(A).

Now, let's continue with the dark on light, e.g., black on white scheme,and another advantageous feature in accordance with certain embodimentsis that the brightness of the “white” background can be adjustable tocontrol the luminance of the target. Through the brightness adjustment,the amount of light entering the test subject's eye is controlled, andthereby the pupil opening is controlled when such viewing target isused.

A disk, which may be a dark disk such as a black disk, is placed at thecenter. This central area can be used as a base of the thin lines. Theboundary of the disk may be curved or may include a series of linesegments or a combination thereof. The point of contact between the thinlines and the disk surface generally the disk surface perpendicular toeach of thin lines in FIG. 3(A). This is advantageous to provide imagediffusion between the base area and the thin line as effective as thetarget shown in FIG. 1. However, the line may form an acute angle withthe base, and there may be multiple lines protruding from the base atdifferent angles even when the base comprises a flat, straight surface.

The exact location of the entire target is not a limiting factor, aslong as it is within the field of view of the test subject.

The exact dimension and shape of the disk at the center is also not alimiting factor of the invention. It may have a diameter from 20/25 RLW,to greater than 20/100 RLW. It can be smaller or larger. When a viewingtarget in accordance with certain embodiments is used at a distance 20feet or 6 meters from the test subject, an advantageous range of thediameter of the disk may be 5 mm to 60 mm.

The length of the thin lines and the radius of the disk are scaled incertain embodiments depending on the viewing distance where the targetis place from the test subject. This scaling feature may be in principlesimilar to that of Snellen eye chart. In its way, the letter size may bedetermined in relationship to the viewing distance.

In the example target shown in FIG. 3(A), the base is a disk having aradius of 20 mm, and the thin lines are 9 mm in length. The “width” ofthe line in the thin dimension is 0.5 mm. The thin line width can rangefrom 0.1 mm to 2 mm, for example. The ratio of the length to the widthof the thin lines ranges from 2 to 1 for the short lines, to the longthin lines with a dimensional ratio of 50 to 1. The dimensional ratio ofthe length to the width of the thin lines is not a limiting factor.

A typical test procedure for determining a visual acuity level of a testsubject can be according to the following: A standard calibrationprocedure may be performed to compare the visual acuity of a standardSnellen eye chart against that of a viewing target in accordance withcertain embodiments. This is recommended since the room light conditionsand the viewing distance may vary from exam room to exam room. The linelength of the viewing target may initially be set to 1.8 mm. This wouldbe equivalent to a 20/20 acuity level. Now we can use this line lengthas a reference.

As specified with the dimensions specified for FIG. 3(A), when thattarget is placed at 20 feet or 6 meters from the test subject, a personwith a visual acuity level of better than 20/20 would be able to see allthe thin lines “sticking” out of the central base area. Then in FIG.3(B), the lines are shortened from 9 mm to 4 mm. Now the test subjectmay indicate that all the lines are still visible. Then the lines arefurther shortened to 2 mm. At this point, the test subject may indicatethat some of the thin lines are missing around the base. FIG. 3(D)illustrates an example of what a test subject may see of the target withsix of the eight lines visible and two not visible to the test subject.The target is showing an acuity standard of 20/(2/1.89)×20, or 20/21.2.The boundary surface of the base provides for blur to wash out the short2 mm line appearance when the line length approaches the limit of thetest subject's visual acuity level. Therefore, such viewing target canbe used effectively to measure the visual acuity level of a testsubject. The acuity level of the test subject in this case, would be20/21.2.

Since the entire target is compact in size, a test subject can see it inits entirety in a glance. It may be compared to looking at a singleSnellen letter in a regular test eye chart. An advantage of a viewingtarget in accordance with certain embodiments, such as that illustratedat FIGS. 3(A) to 3(D), is that it eliminates the directional preferenceof Sellen letters, because it has lines pointing at the entire 360degrees, in 45 degrees steps. The target may include more or less thinlines. The lines can be made to point in 5 degrees, 10, 15, or 30 degreesteps. The angle separation between the lines is not a limitation of theinvention.

One advantage of a viewing target in accordance with certain embodimentsis its ability to measure an approximately continuously variable length(limited only by the pixel size of a display or by an exam room schedulelimitation on number of slides used, and certainly not nearly as limitedby discrete step size as a Snellen eye chart of 20/20, 20/40, etc. Aviewing target in accordance with certain embodiments may be displayedusing an electronic computer monitor, television screens, LED, LCD,Plasma, or electron gun screens, or a projection screen, or anon-electronic display such as a series of slides, cards, wall hangings,or it may be a series of images, printed on paper, or displayed throughelectronic monitor. The line length is easily adjustable using computergenerated graphics. The intensity of the light background may beadjustable by the brightness control of the computer or TV monitor orusing a computer program and the click and/or drag of a mouse.

The number of thin lines and the angular arrangement of the lines aroundthe base are not limiting factors. A sufficient number of lines isdesired in certain embodiments to cover the orientation ranging from 0degree to 180 degrees as shown in FIG. 4(A), although even the singleline embodiment of FIG. 1 may be used advantageously in embodimentswhether the viewing target of FIG. 1 is rotatable to change the angle ofthe line on a display or not. An example of five lines covering 45degree increments from 0 to 180 degrees may be used. FIG. 4(A) showsseven angularly equally spaced lines each covering 30 degree angle stepsfrom zero to 180 degrees. Typical target may contain 5 to 26 lines per180 degrees. But the angular separation may be as small as 2 degreeincrements. It is sufficient to show the target with thin lines coveringonly a semi-circular base as shown in FIG. 4(A), since it covers 0 to180 degrees. However, it may be desired to have a more symmetric andcomfortable viewing target that includes lines protruding over an entirecircular area from 0 to 360 degrees, as shown in FIG. 4(B).

So far, the viewing target has been illustrated with a white background,thin black lines and a black base. However this “black on white” schemeis not a limiting factor. Alternative, one can have the viewing targetin “reverse color”, namely, thin white lines and white base on a blackground, with a white disk base area. Furthermore, the background and thelines/base may have colors such as yellow, green, or red, etc. The colorof the lines, base, and its background is not a limiting factor. Thebrightness level of a white or otherwise light background may becontrolled to display in various gray levels.

Furthermore, the contrast of the black line against the white backgroundmay also be adjustable. If one defines a 100% contrast level as a fullblack on a 100% brightness white background, then one may increase thebrightness of the line to 50% full brightness level, on a 100%brightness white background and define that line as having a 50%contrast level. Now that line is more like a gray line rather than ablack line. In this way, one may adjust the contrast level of the lineto perform contrast acuity testing.

A contrast acuity test may be generally considered more revealing aboutthe quality of vision than a high contrast (100% contrast) acuity test.One may use a viewing target in accordance with certain embodiments toconduct a variable contrast acuity test. To do that, one may first lowerthe contrast level of the line to a desirable level, for example at 12%,or a 5% contrast. Then this lower contrast target may be used to conductan acuity test the same way as using 100% contrast lines, as describedabove. The contrast level of the lines may have a range from 1% to 100%.

Viewing Target as Used in Refraction Procedures

A viewing target in accordance with certain embodiments is moreadvantageous than a Snellen letter eye chart, particularly when it isused to refract, namely to find the best refraction end points for atest subject's sphere, cylinder and/or axis angle.

With a proper arrangement of the thin lines, with equally and angularlyspaced lines, for example, the target eliminates the angular asymmetryor the angular preference of a letter eye chart. Such target issensitive for cylinder errors and at cylindrical axis.

The thin lines are sensitive to the effects of spherical error, ordefocusing errors. Their widths are smaller than those of a typicalstandard Snellen letter. Over-minusing is a condition that is oftenencountered when Snellen eye charts are used. Due to the thin linethickness of the lines of the viewing target in accordance with certainembodiments, there is no demagnification effect in the line widthdimension, which could happen with Snellen letters. When too much minusspherical power is added to the refraction correction, the size of theletters of the Snellen eye chart appear to be darker due to thereduction in letter size with higher minus power, leading to a falserefraction end point. Since the line is thin, there is very littleeffect to produce a thinner line, or “darker” appearance, as in the casewith Snellen letters. Therefore, a viewing target in accordance withcertain embodiments does not favor over-minusing as would be when usinga Snellen eye chart.

When there is residual cylinder error, a target as shown in FIG. 5(A)may appear to the test subject as shown in FIG. 5(B), in which some ofthe lines would disappear while other lines at 90 degree from them stayvisible and appear more elongated. When cylinder error is corrected, thethin lines appear equally visible and focused as shown in FIG. 5(A).Therefore, one way to determine if there are any residual cylindererrors is to shorten the thin lines from FIG. 5(A), to FIG. 5(C), thento FIG. 5(D), and ultimately to a shortened length when all linesdisappear almost simultaneously at about the same line length. Snelleneye charts cannot provide such a definitive end point by looking at theletters.

To find an axis angle of a cylinder error, one could use the partialdisappearance of the lines to determine which angle is closer to theaxis end point, namely, at the corrected axis angle. For example, if thetarget appears to the test subject as shown in FIG. 5(A) at axis angleX, as compared to an image of FIG. 5(B) formed at a second axis angle Y.In this case, the corrected axis angle is closer to the angle X ascompared to angle Y. By comparing the image quality of a viewing targetin accordance with certain embodiments at two cylinder angles underconsideration, one can use the quality of the image of all lines todetermine which one is more symmetric and more focused, therebyapproaching the optimal end point for the cylinder correction.

There are other variations of viewing targets in accordance withalternative embodiments. For example, one thin line may be made toappear at a time, as illustrated at FIG. 6. Using an electronic display,one may display only one line at any instant in this embodiment. Theposition of that line then shifts to the next incremental angle asillustrated at FIG. 6. A line 610 first appears. Then line 610disappears, and line 620 appears at 15 degrees, for example, angularlyspaced from line 610. Next the line 620 disappears, and new line 630appears at another 15 degrees from line 620. This disappearance andappearance sequence continues with a new line at an angular increment,as new lines appear around the base disk, as illustrated in FIG. 6. Eachof the lines may appear for a duration ranging 0.01 seconds to 2seconds. The stay on time of the line is generally not a limitationhere. The angular separation of 15 degrees here is mere exemplary, andnot a limitation either. The angular steps may range from 2 degrees to45 degrees.

Another variation can be that the base area is a square as shown in FIG.7, or a hexagon as in FIG. 8, or an octagon as in FIG. 9, or some otherpolygon or curved regular or irregular shape. The thin lines radiatefrom the base edges. The square can be changed to a rectangle. Rounddisks of the base may be substituted by an oval base area. The exactshape of the base is also not a limiting factor.

The described embodiments are merely illustrative and the invention isnot limited to the specifically-described examples. Instead, theinvention is set forth in the claims and includes structural andfunctional equivalents thereof.

What is claimed is:
 1. A non-transitory computer readable tangiblemedium configured for programming a processor to produce on a display aviewing target for a visual acuity or a refraction measurement of a testsubject, wherein the viewing target comprises: (a) at least one linecomprising a width dimension that is within a range that includes aresolution limit width (hereinafter “RLW”) of a test subject visualacuity, and an adjustable length dimension that is initially set withinsaid range that includes said RLW of the test subject visual acuity andis adjustable to other values; (b) a base, comprising one or morefurther lines including at least one of said one or more further linesapproximately intersecting said at least one line, wherein a number ofsaid one or more further lines is adjustable; (c) wherein the basecomprises a round, elliptical, circular, semi-circular, oval, square,rectangular, hexagonal, octagonal or other polygonal, or curved regularor irregular shape; and (d) wherein each of one or more of color,brightness, or gray level of the at least one line or of the one or morefurther lines, or both, is adjustable; and (e) wherein the one or moreof the color, brightness, or the gray level of the background withinwhich the viewing target is displayed is also adjustable; and (f)wherein at least one line of the viewing target changes position inangular increments by appearing at a first position for a temporalduration selectable between 0.01 seconds and 2 seconds, and thendisappearing from the first position and appearing at a second positionthat is displaced from the first position by an angular increment; and(g) wherein the angular increments are selectable between 2 degrees and45 degrees.
 2. The tangible medium of claim 1, wherein the at least oneline comprises multiple lines protruding from the base, each of themultiple lines are separated by an angular displacement between 2degrees and 45 degrees.
 3. The tangible medium of claim 2, wherein eachof the multiple lines differs either in length or direction, or both. 4.The tangible medium of claim 3, wherein the base comprises a polygon andeach of the lines protrude from the base, and point at selectabledirections.
 5. The tangible medium of claim 3, wherein the viewingtarget comprises at least five lines protruding from the base angularlyspaced at approximately equal angles apart, wherein the angles rangefrom 2 degrees to 45 degrees.
 6. The tangible medium of claim 3, whereineach of the lines are equal in length and adjustable.
 7. The tangiblemedium of claim 3, wherein the base comprises a circular shape.
 8. Thetangible medium of claim 3, wherein the target is adjustablyelongatable.
 9. The tangible medium of claim 2, wherein the angulardisplacement between multiple lines has equal angle value or each of theangular displacements has an adjustable value, selectable between 2degrees and 45 degrees, or both.
 10. The tangible medium of claim 1,wherein the angular increments are equal, or the angular increments areadjustable, or both.
 11. The tangible medium of claim 1, wherein thetemporal durations are equal, or the temporal durations are adjustable,or both.
 12. A visual acuity measurement system, comprising: (a) adisplay for showing a viewing target as in claim 1; (b) a machinecoupled with the display and configured to adjust the viewing target onthe display during a visual acuity measurement of a test subject; and(c) a calculator or look-up table, or both, for determining the visualacuity of the test subject based on the adjusting of the viewing target.13. A non-transitory computer readable tangible medium configured forprogramming a processor to display a viewing target for a visual acuityor refraction measurement of a test subject, wherein the viewing targetcomprises: (a) at least one line comprising a width dimension that isaround a resolution limit width (hereinafter “RLW”) of a test subjectvisual acuity, and an adjustable length dimension that is initially setaround the RLW of the test subject visual acuity and is adjustable togreater than the RLW; (b) a base, comprising at least one or morefurther lines including at least one of said one or more further linesapproximately intersecting the at least one line, wherein the viewingtarget includes a round, elliptical, circular, semi-circular, oval,square, rectangular, hexagonal, octagonal or other polygonal, or acurved regular or irregular shape; and (c) wherein the at least one lineof the viewing target is configured to appear and disappear to the testsubject during said measurement.
 14. The tangible medium of claim 13,wherein the direction at which the at least one line is adjustablewithin a range of different angles between 2 degrees and 45 degrees. 15.The tangible medium of claim 13, wherein temporal increments betweenappearing and disappearing of the viewing target to the test subjectcomprise between 0.01 seconds to 2 seconds.
 16. The tangible medium ofclaim 13, wherein the line and base comprise a same color.
 17. Thetangible medium of claim 13, whereby in performing a visual acuitymeasurement of a test subject, the length dimension of the line isadjustable in increments small enough to effectively approximate thevisual acuity of the test subject by determining a shortest resolvableline and a next smaller line that is not resolvable by the test subject.18. The tangible medium of claim 17, wherein the increments of theadjustable length of the line comprise an integer multiple of onedisplay pixel.
 19. The tangible medium of claims 13, wherein the atleast one line and the base each comprise black color, and a backgroundagainst which the target is displayed comprises white or gray color, ora combination thereof.
 20. The tangible medium of claim 13, wherein thewidth of the at least one line comprises less than 20/10 RLW.
 21. Thetangible medium of claim 13, wherein the length of the at least one linecomprises greater than 20/40 RLW.
 22. The tangible medium of claim 13,wherein the thickness of the base comprises greater than 20/40 RLW. 23.The tangible medium of claim 13, wherein the at least one line comprisesa plurality of lines differing in their length or direction, orcombinations thereof.
 24. The tangible medium of claim 23, wherein thebase comprises a polygon and each of the plurality of lines protrudesfrom the base, and point at selectable directions.
 25. The tangiblemedium of claim 23, comprising at least five lines protruding from thebase angularly spaced at approximately equal angles apart, wherein theangles are in a range from 2 degrees to 45 degrees.
 26. The tangiblemedium of claim 23, wherein the plurality of lines are equal in lengthand adjustable.